For producing sheetlike metallic workpieces such as, for example, automobile parts, bodywork parts, instrument casings, exterior architectural facings, ceiling panels or window profiles, suitable metal sheets are shaped by means of appropriate techniques such as punching, drilling, folding, profiling and/or deep drawing. Larger components, such as automobile bodies, for example, are assembled if appropriate by welding together a number of individual parts. The raw material for this purpose normally comprises long metal strips which are produced by rolling the metal and which for the purposes of storage and transportation are wound up to form what are called coils.
Whereas in the past the corrosion control treatment was essentially carried out on the finished metallic workpiece—an automobile body assembled by welding, for example—nowadays the corrosion control treatment is increasingly being performed by coating the coil metal itself.
This technique is also referred to frequently as “coil coating”. In coil coating the coif metals are coated continuously with—in general, liquid—coating materials. Metal coils 0.2 to 2 mm thick and up to 2 m in width, made of steel or aluminum, for example, are transported at a speed of up to 200 m/min through a coil coating unit, in the course of which they are coated with one or more corrosion control and/or paint layers.
A typical coil coating operation normally comprises the following steps:    1. If necessary: cleaning of the metal coil to remove contamination which has attached itself during the storage of the metal coil, and also to remove temporary corrosion control oils, by means of cleaning baths.    2. Application of a thin pretreatment layer (<1 μm) by dipping or spraying or by roll application. This layer is intended to increase the corrosion resistance and serves to improve the adhesion of subsequent paint layers to the metal surface. Known for this purpose are both Cr(VI)-containing and chromate-free pretreatment baths.    3. Application of a primer by a roll method. The dry layer thickness is normally about 5-8 μm. Solvent-based coating systems are used here.    4. Application of one or more topcoat layers by a roll method. The dry layer thickness here is approximately 15-25 μm. Here, likewise, solvent-based coating systems are employed.
The layer structure of a galvanized steel coil coated in this way is depicted in FIG. 1. The figure shows a section through a steel coil (1) which has been provided with a zinc layer (2) and to which a conventional pretreatment layer (3), a primer coat (4), and a topcoat (5) have been applied.
Metal coils coated in this way are employed, for example, to produce casings for what are called white goods (refrigerators, etc.), as facing sheets for buildings, or else in automotive engineering.
As set out above, the coating of the metal coils with the pretreatment layer (3) and with a primer coat (4) is very costly and inconvenient. Furthermore, market demand is on the increase for Cr(VI)-free systems for corrosion control. Proposals have therefore been made to replace separate application of a pretreatment layer (3) and of the organic prime coat (4) by application of a single, integrated pretreatment layer (3′) which takes on the functions of both layers. A layer system of this kind is shown by way of example and diagrammatically in FIG. 2. The production of a coated metal coil is greatly simplified by a one-stage operation of this kind.
U.S. Pat. No. 5,322,870 discloses a composition for forming an integrated pretreatment layer, comprising a polymeric coating material, a crosslinker, and, in addition, alkyl- or arylphosphoric esters and/or alkyl- or arylphosphonic esters. The composition may optionally also comprise a pigment.
DE-A 199 23 084 discloses a chrome-free aqueous coating material for one-stage coating, the material comprising at least hexafluoro anions of Ti(IV), Si(IV) and/or Zr(IV), a water-soluble or water-dispersible film-forming binder, and an organophosphoric acid. The composition may optionally also comprise a pigment and crosslinking agents.
EP-A 878 519 discloses preferably chrome-free, aqueous compositions for producing corrosion control coatings, comprising 0.2 to 50 g/l of a thiocarbonyl compound, 0.1 to 5 g/l of phosphates, and water-soluble binders or binder dispersions. Optionally 10 to 500 g/l of SiO2 may be present. The thiocarbonyl compounds may be, for example, thiourea, thioamides, thioaldehydes or thiocarboxylic acids.
DE-A 4 308 214 discloses a process for preparing dialkyl dithiophosphinates and also their use for ore extraction. U.S. Pat. No. 5,872,279 discloses the use of bis(1,1,3,3-tetramethylbutyl)thiophosphinic acid as an extractant for metals.
Homer, L; Hinrichs, H in Werkstoffe and Korrosion 1971 22, 864-868 disclose the use of diphenylthiophosphinic acid as a corrosion inhibitor for iron wires and iron powders in aqueous hydrochloric acid at a variety of concentrations.
Chromy, L; Marek, K, DEFAZET—Deutsche Farben-Zeitschrift 1978, 32, 434-437 disclose the use of diphenylthiophosphinic acid in a mixture with a vinyl chloride-vinylidene chloride copolymer or with a vinyl chloride-isobutyl ether-polyvinyl alcohol copolymer for coating steel in layer thicknesses of from 30 to 70 μm. When diphenylthiophosphinic acid was added to the polyvinyl alcohol polymers a deterioration was found in activity in comparison to the unadditized polymer.